The recent introduction of cellular telephone service which utilize frequencies in the 800 MHz frequency band and above, has increased interest in the efficient mobile antenna systems for those fequencies. Such services typically utilize a fairly wide band width. For example, existing and/or proposed systems operate over frequency bands of about 800-870 MHz, 820-900 MHz and 860-940 MHz. As can be seen by the above figures, the band width of such operating systems ranges from between about 60 to about 80 MHz. Thus, any antenna designed for use with such systems should provide efficient radiation characteristics and low VSWR over these band widths.
In addition, mobile antennas for such communications systems are designed to be mounted on vehicles. Some type of permanent installation is often necessary. For preferred locations, those which provide the most uniform radiation patterns, such as roof tops, this requires mounting to the vehicle such as automobiles by cutting holes into the body and permanently mounting the antennas in place. This is not always a satisfactory arrangement for vehicle owners.
Alternate mounting locations, such as fenders or trunk lids, which may allow for different mounting techniques, result in deterioration in the desired uniformity in the radiation pattern. It would be desirable, therefore to have an antenna which could operate at these UHF frequencies and which at the same time could provide the desired operating characteristics without requiring the mounting arrangements that permanently mar a vehicle and require body repair when the antenna system is removed from the vehicle.
The mounting of a communications antenna on insulated surfaces such as the windshield of an automotive vehicle is known for much lower frequencies. One such an antenna system is disclosed in commonly assigned U.S. Pat. No. 4,238,799 which issued on Dec. 9, 1980, incorporated herein by reference.
The antenna system there specifically disclosed is particularly adapted for operation at frequencies well below the frequencies used for cellular phone communication systems. Thus, the antenna there disclosed was designed for operation in the CB and related bands of about 28-29 MHz.
Antennas similar to and adapted from the antenna disclosed in the aforesaid U.S. Pat. No. 4,238,799 have been designed and operate at somewhat higher frequencies than those disclosed in that patent. However, although the electrical schematic representation of the circuit remains the same as that shown in FIG. 4 of that patent, as frequencies increase and reach the frequencies utilized in cellular phone systems, those at and above the 800 MHz band, the structure utilized for lower frequencies is no longer appropriate.
Furthermore, the antenna disclosed in the aforesaid patent is a relatively narrow band antenna which does not operate satisfactorily over the wide frequency bands which are required for cellular phone systems.